The present invention relates to a method for manufacturing a productive organic electroluminescence device with high luminous efficiency and an organic electroluminescence device.
An organic electroluminescence device (hereinafter, referred to as an organic EL device), which is a self-emitting display device having features such as fast response, independence of viewing angles, and low power consumption, is receiving attention as a device for next-generation displays.
At present, an area color mode in which monochrome organic EL devices are locally combined is applied to display panels for in-car audio, and a full color mode in which red (R), green (G), and blue (B) organic EL devices are patterned by mask evaporation and the above-mentioned area color mode are applied to display panels for mobile phones.
Under such a circumstance, further research and development are being made to manufacture organic EL devices which are driven at low voltage and provide high-intensity light emission and high color reproduction.
In a typical method for manufacturing an organic EL device, for example, a light-emitting layer composed of an organic material is formed on a transparent substrate on which a film of transparent indium-tin oxide (ITO) or the like is formed as an anode (transparent electrode), and then a cathode (metal electrode) of Al, Ag, or the like is laminated thereon.
For the organic material of the light-emitting layer in this case, a wide range of materials are available including, for example, fluorescent conjugated and non-conjugated polymer materials, fluorescent small-molecular materials, fluorescent metal complexes, and further, phosphorescent heavy metal complexes and the like which emit light with very high luminous efficiency. At this time, for the method for manufacturing an organic EL device, a wet method such as application of solution or a dry method such as vacuum evaporation is selected depending on the type of an organic material to be used.
Generally, the organic EL devices are classified into a single layer type composed of a single light-emitting layer and a laminate type including a charge injection layer, a charge transport layer, a light-emitting layer, an electron injection layer, and the like which are sequentially laminated by function, using a plurality of different organic materials. In any type thereof, light emitted from the light-emitting layer can be extracted to the outside through the transparent substrate on which the transparent anode is formed or through the transparent cathode.
The Japanese Patent Publication No. 2000-91078 (page 3, FIG. 1) discloses an organic EL device including an anode, a light-emitting layer, an electron injection layer, and a cathode sequentially formed on a substrate. In this organic EL device, the electron injection layer is formed of an organic salt or an organic metal complex of an alkali metal or a Group 2 metal in order to lower turn-on voltage and increase light emission intensity.
FIG. 1 is a cross-sectional view of an organic EL device according to the above-mentioned prior art (hereinafter, referred to as prior art 1). In an organic EL device 10A shown in FIG. 1, a hole transport layer 3 and a light-emitting layer 4 are sequentially laminated on an anode (transparent electrode) 2 composed of transparent ITO or the like which has been formed on a transparent glass substrate 1. Further, as a cathode 5, a Group 2 metal 5A such as Ca or Mg and a low electric resistance metal 5B such as Al or Ag are sequentially laminated thereon. Subsequently, a desiccant 7 is attached to the inside of a top part 6a of a cap 6 which is formed into a cup shape using glass, a SUS material, or the like. This cap 6 covers the light-emitting layer 4 and the hole transport layer 3 from above the cathode 5. The bottom edge of the cap 6 is fixed onto the anode 2 and the glass substrate 1 with a UV-curing resin 8 interposed therebetween.
The Group 2 metal 5A includes a function to inject electrons into a light-emitting layer 4 side because the Group 2 metal 5A has a low work function. The low electric resistance metal 5B has a resistivity lower than other metals (for example, the resistivity of Al and Ag are 2.66×10−6 Ωcm and 1.59×10−6 Ωcm, respectively) and allows an electric current to easily flow. Accordingly, the turn-on voltage is set lower. This is because the prior art 1 uses the combination of the Group 2 metal 5A and the low electric resistance metal 5B as the cathode 5.
FIG. 2 is a cross-sectional view of an organic EL device according to another prior art (hereinafter, referred to as prior art 2). An organic EL device 10B of FIG. 2 is different from the organic EL device 10A of the prior art 1 only in the structure of the cathode 5. Specifically, the cathode 5 of the prior art 2 is formed as a mixture obtained by co-evaporating the Group 2 metal 5A such as Ca or Mg with a low work function or an alkali metal 5C such as Cs or Li with an extremely low work function, and the low electric resistance metal 5B such as Al or Ag.